The purpose of this study is to examine the host immune response to viruses. The major goal is to examine the normal immune response to naturally occurring viruses in man and to extend these studies to patients in order to identify abnormalities of immune regulation which may be related to the pathogenesis of certain diseases of the nervous system. These studies involve the examination of various T cell subsets and the role of soluble factors, such as IL-2 and interferon in the response to viruses including measles virus. T cell lines and clones are also being used to examine cellular reactivity to these viruses. Studies have also examined the host immune response in experimental infections and are investigating the role of these responses in acute or chronic infections of the nervous system.